Thread Repair Insert Calculator: HeliCoil Drill & Tap Sizing Guide

How It Works

1. Identify the Damaged Thread

   Determine the original thread size from the supported UNC or Metric rows, with diameter and pitch. Use a thread pitch gauge or the original bolt specifications.

2. Set Material and Load

   Choose the host material and application load. The calculator recommends an insert length from 1.0D to 3.0D (the greater of the load guideline and the material minimum), expressed as a multiple of the nominal thread diameter.

3. Review Drill Prompt

   The calculator shows a commonly published oversize drill prompt for boring out the damaged thread ahead of the STI (Screw Thread Insert) tap. Verify against the insert manufacturer installation instructions before drilling.

4. Review STI Tap Size

   Use the STI tap prompt as a catalog-check item. STI taps cut the oversize thread that accepts a wire insert and are not the same as standard taps.

5. Install and Verify

   Drill, tap, install the selected insert, and verify with the insert manufacturer instructions and appropriate thread gauge before service.

Built For

• Maintenance mechanics repairing stripped bolt holes in aluminum castings (engine blocks, gear cases, pump housings)
• Aviation mechanics checking HeliCoil prompts against manufacturer specifications
• Machine shop operators repairing threaded holes in fixtures and workholding
• Automotive technicians reviewing stripped spark plug holes, head bolt holes, and oil pan threads before catalog selection
• Fabricators comparing thread insert length prompts for soft materials
• Field service technicians carrying a thread repair kit with pre-identified drill and tap prompts
• Manufacturing engineers screening thread insert options before manufacturer data review

Features & Capabilities

UNC and Metric Coverage

Common thread sizes: UNC #4-40 through 1"-8 (15 rows, including #10-32) and Metric coarse M3 through M20 (10 rows). UNF beyond #10-32 and metric fine pitches are not included.

Material- and Load-Driven Insert Length

Screens 1.0D, 1.5D, 2.0D, or 3.0D from the host material minimum and application load. A reference table shows all multiples side by side for catalog review.

Drill Prompt Lookup

Shows the commonly published STI tap drill for each size (number/letter/fractional for UNC, millimeters for metric) alongside the standard tap drill for comparison.

STI Tap Identification

Flags the Screw Thread Insert tap prompt for each supported wire-insert row. STI taps are thread-specific and must match the insert catalog.

Depth Calculator

Screens minimum hole depth from insert length plus pitch clearance; counterbores, key-locking features, and manufacturer-specific prep remain outside the calculation.

PDF Export

Export insert sizing prompts as a branded PDF for repair work orders or kit inventory reference.

Assumptions

• Insert drill and tap rows are local commonly published prompts and were not reconciled to current manufacturer catalogs in this pass
• STI (Screw Thread Insert) tap dimensions are catalog-controlled and must match the selected insert system
• Insert length expressed as multiples of nominal thread diameter: 1D, 1.5D, 2D, and 3D
• Thread pitch and major diameter prompts reference supported UNC and metric coarse rows only
• Blind hole depth screen assumes 2 thread pitches of clearance below the estimated insert length
• Pullout is a generic thread-shear calculator, not a tested insert rating or safety-factor calculation

Limitations

• Insert part numbers are not generated; actual part numbers vary by manufacturer and may include locking, free-running, or screw-lock variants
• Does not calculate parent material minimum wall thickness required around the oversize drilled hole
• Tangless vs tanged insert selection depends on application requirements and is not automated
• E-Z Lok and Keensert selections provide comparison notes and installation guidance only; their system-specific drill, tap, and counterbore prep is not computed and must come from those catalogs
• UNF beyond #10-32 and metric fine pitch rows are not included
• Does not account for thread locking inserts, which use a different internal thread form and require specific installation tooling

References

• STANLEY Engineered Fastening Heli-Coil technical data - manufacturer source pointer for catalog verification
• ASME B1.1 - Unified Inch Screw Threads source pointer for inch-thread context
• ASME B1.13M - Metric M-profile screw threads source pointer for metric-thread context
• NIST SP 811 Appendix B.8 - unit conversion source pointer

Frequently Asked Questions